The present invention relates to thermoplastic elastomer compositions that are suitable for application to gearshift knobs, assist grips, armrests and other automotive interior features that require superior flexibility, wear resistance, oil resistance, and desirable appearance.
Recently, thermoplastic elastomer resins that are rubber-like, but do not require curing, and have workability similar to that of thermoplastics have been receiving much attention for applications such as automotive, home appliances, and construction materials. Furthermore, vinyl chloride resin-based elastomers that have been traditionally used for automotive interior materials are suspected as containing environmentally harmful plasticizers and sources of dioxin when burned. Accordingly, the movement towards replacing such materials with more environmentally friendly, non-vinyl chloride elastomers is becoming increasingly popular. Because automotive interior materials frequently come into close physical contact with human hands, the point in developing alternative materials is to bring the wear resistance as close as possible to that of vinyl chloride materials. In the case of thermoplastic elastomers, it is well known that the wear resistance increases if the hardness is made extremely high, but that does not provide the feel of vinyl chloride materials; therefore, the development of more flexible and fairly wear resistant non-vinyl chloride elastomers is sought.
Polyurethane-based resins are known to have superior wear resistance, but are not desirable as environmentally friendly materials because they produce hazardous gases when burned. They also have inferior forming stability.
Attempts also have been made to add an organic lubricant, such as a higher fatty acid amide or low viscous silicone, to olefin-based and styrene-based thermoplastic elastomers in order to increase the surface smoothness of formed articles, and thereby improve their wear resistance. However, because these materials migrate to the surface of the formed articles over time or through heat, the appearance of the formed articles is adversely affected and the expected wear resistance is not achieved.
An object of the present invention is to avoid the above problems associated with conventional techniques and provide thermoplastic-resin compositions that can be used as automotive interior materials, and which are more flexible and wear resistant, have good oil resistance and pleasing appearance, and do not contain chlorine and therefore are environmentally friendly.
The above and other objects of the present invention can be achieved by a thermoplastic elastomer resin composition comprising
(a) 100 parts by weight of a block polymer composed of at least two polymeric blocks A (primarily vinyl aromatic compounds) and at least one polymeric block B (primarily conjugated diene compounds), wherein said vinyl aromatic compound content falls within the range of 5-70% by weight and 70% or more of said conjugated diene compound portion is hydrogenated;
(b) 50-170 parts by weight of a polyolefin-based resin whose melt flow rate (ASTM-D-1238L condition at 230xc2x0 C.) is 1-40 g per 10 minutes;
(c) 100-300 parts by weight of a non-aromatic plasticizer for rubber;
(d) 50-170 parts by weight of a thermoplastic elastomer composition containing crystalline polyolefin and completely or partially crosslinked ethylene-xcex1-olefin polymer, wherein said xcex1-olefin polymer density falls in the range of 0.858-0.915 g/cm3 and the molecular weight distribution (M/Mn) (weight average molecular weight (M) over number average molecular weight (Mn)) is less than 3.0; and
(e) 5-50 parts by weight of silicone oil whose viscosity (JIS Z8803 at 25xc2x0 C.) is 50,000 centistoke or more.
It is desirable from the above defined thermoplastic elastomer resin composition to contain an ethylene-xcex1-olefin polymer in the constituent (d) which is manufactured by using a metallocene-based catalyst.